Traditionally the technology for the fabrication of semiconductor devices depends on lithographic micropatterning using photoresist compositions. Integration of semiconductor devices is effective in improving the performance, function and reliability of devices and reducing the cost. Since the birth of integrated circuits (IC), technical development works have been continued to boost the integration level.
With the target on further device integration, efforts have been made to reduce the pattern rule by reducing the wavelength of exposure light in photolithography. In the mass production process of 64 M-bit DRAM, for example, the exposure light source of i-line (365 nm) was substituted by a KrF excimer laser having a shorter wavelength of 248 nm. For the fabrication of DRAM with an integration density of 1 G or more requiring a finer patterning technology (feature size 0.13 μm or less), photolithography using ArF excimer laser (193 nm) is under investigation.
Increasing the diameter of lens and increasing the numerical aperture (NA) of lithography system by adopting immersion lithography are also effective in reducing the pattern rule. However, as the NA increases, reflection of exposure light by a substrate poses a greater impact. Specifically, a process of providing a bottom antireflective coating (BARC) between a photoresist film and a substrate is generally employed in order to control reflection. Prior art ARCS are difficult to fully avoid the reflection of exposure light by the substrate. This raises a problem that exposure variations, known as standing waves, occur in a resist film thickness direction, resulting in a decline of resolution.
Known prior art ARCs include inorganic ARCs such as Si and TiN, and organic ARCs composed of light absorber-loaded polymers. The inorganic ARCs require deposition equipment such as vacuum evaporation, CVD and sputtering systems whereas the organic ARCs can be formed in a relatively simple manner without a need for any special equipment. The organic ARCs are not only required to suppress the reflection of exposure light, but also required to have a high dry etching rate as compared with resist film and to avoid intermixing with resist film and diffusion of low-molecular-weight fraction into the resist film (see SPIE Vol. 2195, p 225-229 (1994)).
While the immersion lithography is adapted to insert a liquid between the projection lens and the wafer, the use of water having a refractive index of 1.44 at wavelength 193 nm enables to design a projection lens to a numerical aperture (NA) which is 1.44-fold higher than the lithography in air with a refractive index of 1.0. The immersion lithography, when combined with a projection lens having a NA around 1.3, enables pattern formation at a half pitch of 45 nm.
As the NA increases, the incident angle of light on the resist film becomes shallower, with more oblique light entering. It is pointed out that the proportion of oblique incident light is further increased by s-polarized illumination intended for contrast enhancement, so that reflection by the substrate is further increased. In contrast, it is reported that bi-layer BARC(SPIE Vol. 6153, p61531J (2006)) and a tri-layer process including a silicon-containing intermediate film and an undercoat film having an antireflection effect (SPIE Vol. 6153, p61530K (2006)) have a high antireflection effect at NA 1.0 or above. At NA 1.2 or above, however, the bi-layer ARC is insufficient and a further reduction of reflection is necessary. In contrast, it is reported in SPIE Vol. 6153, p61531J (2006) that a graded BARC which becomes more absorptive toward the substrate side is more effective for preventing reflection than the bi-layer BARC. It is known from the past to use an ARC in the form of a multilayer coating in order to prevent reflection of light over a wide range of wavelength, as implemented on optical lenses and the like. A substrate having a good antireflection effect over a wide range of wavelength can also be an antireflective substrate compliant with varying incident angle. An ARC in which the number of multiple layers is increased infinite, that is, absorption is changed stepwise can exert an excellent antireflection effect. One example that applies this principle to BARC is described in JP 3414107. The illustrative materials used to provide stepwise absorption changes are photo-bleachable materials such as nitrone.
A multilayer ARC may be formed by iterative application of BARCs with different coefficients of absorption, but this process is impractical because of process complexities and reduced throughputs. It is preferred from the standpoint of process simplicity that an ARC having graded absorption be formed by a single coating step. Although the photo-bleachable materials are proposed in JP 3414107, no such materials are available at wavelength 193 nm. It is then difficult to design an ARC which increases its transparency on the resist side upon exposure.
JP-A 2000-53921 discloses a method of forming a two-layer ARC for reducing the reflection of visible light, using an ARC-forming composition comprising a fluorine-containing compound capable of forming a low refractive index cured film and a compound capable of forming a high refractive index cured film with greater surface free energy. It is believed that self-arrangement and collection of molecules proceed at the film formation stage so that the free energy of film surface may become minimum, and a two-layer structure forms due to a phase-separation phenomenon. This method permits two layers to be formed by a single coating step and is thus effective for both reflectance reduction and productivity improvement. However, if the difference in free energy between two polymers is inadequate, a two-layer structure due to phase separation may not always form and in many cases, a so-called island-in-sea structure forms in which domains of one phase are distributed in a matrix of the other phase. To form an ARC of two-layer structure, an optimum combination of compounds must be sought for.
As described above, the ARC is also required to have a high etching rate. A reduction of pattern feature size entails a thickness reduction of a photoresist film, which encourages to increase the etching rate of ARC. In the past, novolac resins and polyimide resins were used as the polymer to form ARC, but their etching rate is very slow, leaving a problem that the photoresist pattern has been slimmed at the time when the ARC film extinguishes. One effective means for increasing the etching rate is by reducing the number of carbon atoms and increasing a proportion of oxygen atoms. A study was thus made on the use of (meth)acrylate polymer having crosslinking groups and absorptive groups as the base resin. To further accelerate the etching rate, a polyester resin having oxygen atoms incorporated in the polymer backbone was proposed as the base resin.
In the efforts to develop F2 lithography photoresist materials, various fluorine-containing polymers were examined for the purpose of improving transparency at 157 nm. Those polymers having perfluoroalkyl groups had so high water repellency that pattern collapse and stripping occurred due to incomplete development and poor substrate adhesion. On the other hand, alcohols having multiple fluorine atoms substituted at α-position provide appropriate alkaline solubility and adhesion because the hydroxyl group is rendered acidic due to the electron withdrawing nature of fluorine. Thus a number of F2 lithography polymers having —C(CF3)nOH structure were developed.